STUDIES IN SPICULE FORMATION. 



37 



spicules. These plates each originate as a spherical granule 

 contained within a single cell (fig. 19), and this granule 

 gradually becomes larger (fig. 20), and early assumes the 

 plate-like form (fig. 22). There is thus no rod or triradiate 

 stage — so common in the development of echinoderm spicules 

 — in the growth of these plates, and the plate is never of the 

 perforate type. The nucleus of the original mother-scleroblast 

 divides at an early stage of growth of the spicule and the 

 number of nuclei present at different stages of growth (a 



Full-sized imperforate plates of Antedon bifida X cir. 270 

 diameters. These plates are very thin, easily fractured, show con- 

 centric lines (presumably of growth), and are to be found in large 

 numbers in the hypostroma of the integument. 



score or more on fully-developed plates) is strictly propor- 

 tional to the size attained by the spicule. These large thin 

 squamose spicules of Antedon do not seem to be generally 

 known, although they exist in great numbers. They are 

 very easily decalcified in virtue of their extreme thinness, 

 and, as I have already mentioned, are very unlike echinoderm 

 spicules (so much so that I at first mistook them for artefacts) . 

 As Dr. Bather has very kindly pointed out to me, similar, 

 but much thicker, imperforate plates (with no concentric 

 markings) have been described, among others, by Ludwig 

 in Holothurians (4), Ophiuroids (5) (also Mortenson [8]), and 

 Asteroids (Astropectinidte) (6). 



